A gate oxide film, or gate insulating film, is an important, thin insulating film which is part of a TFT. In TFTs, relatively high voltage is generally applied to the gate electrode, which requires the gate oxide film to be completely insulating. The gate oxide film is formed up to a thickness sufficient for this requirement (about a few tens to 100 nm) so that no leak current occurs. If an increase in leak current density is allowed, that will cause additional power consumption in the device, rise in operating temperature, instability, and other numerous problems. Furthermore, if the leak current has increased roughly equal to the drain current, the device is put at risk of being inoperable.
The gate oxide film makes direct impacts, in this manner, on the performance (reliability and characteristics) of various devices containing the TFT. High quality and high reliability is particularly needed with the gate oxide film.
The gate oxide film (for example, silicon dioxide film) in the TFT is typically formed by CVD (chemical vapor deposition). In CVD, an organic silane, for example, tetra-ethoxy-silane (TEOS), is thermally decomposed at a few hundred degrees Celsius so that it deposits on a substrate to form an oxide film (gate oxide film).
There are other known methods, including sputter vapor deposition whereby the oxide is formed by sputter vapor deposition and plasma oxidation whereby the substrate surface is oxidized in plasma.
Another method is anodization whereby the substrate surface is oxidized, forming oxide film thereon, by anodization. Well-known specific examples are found in Japanese Unexamined Patent Publication 3-6826/1991 (Tokukaihei 3-6826; published on Jan. 14, 1991); Japanese Unexamined Patent Publication 52-78374/1977 (Tokukaisho 52-78374; published on Jul. 1, 1977); Japanese Unexamined Patent Publication 2003-133309 (Tokukai 2003-133309; published on May 9, 2003); Applied Physics, vol. 44, issue 5, p.p. 497 to 506, 1975; and Encyclopedia of Electronics Technology, MOS device (1st print in 1973), p.p. 124 to 125, Takashi TOKUYAMA. According to the method, voltage is applied to a silicon substrate in an aqueous solution of hydrofluoric acid (electrolyte) to form a porous anode-reaction silicon film. The porous anode-reaction film is subjected to anodization in an electrolyte in which silicon is anodized, for example, thick phosphoric acid.
In anodization, the voltage application moves silicon ions to the surface of the silicon substrate so that they form a silicon dioxide film on the surface. After the formation of the silicon dioxide film, to promote oxidation reaction at the interface between the formed silicon dioxide film and the electrolysis solution (surface of the silicon dioxide film), silicon ions are generated from the silicon substrate. Typically, a voltage as high as 100 V or more needs to be applied so that the silicon ions can pass through the silicon dioxide film to reach the interface between the silicon dioxide film and the electrolysis solution (surface of the silicon dioxide film). See Encyclopedia of Electronics Technology, MOS device (1st print in 1973), p.p. 124 to 125, Takashi TOKUYAMA.
These are all electrical methods based on voltage application. In contrast, the inventor has suggested chemical methods of forming an oxide film in various literatures including: Japanese Unexamined Patent Publication 2004-47935 (Tokukai 2004-47935; published on Feb. 12, 2004); Japanese Unexamined Patent Publication 9-45679/1997 (Tokukaihei 9-45679; published on Feb. 14, 1997); Japanese Unexamined Patent Publication 2002-57154 (Tokukai 2002-57154; published on Feb. 22, 2002); Japanese Unexamined Patent Publication 2002-64093 (Tokukai 2002-64093; published on Feb. 28, 2002); J. Applied Physics Letters, 81, 18, p.p. 3410-3412 (2002); and J. Applied Physics Letters, 94, 11, p.p. 7328-7335 (2003). For example, in Tokukai 2004-47935, the inventor suggests to form an oxide film as thin as about 1 nm on a surface of a silicon or other semiconductor substrate using thick nitric acid or a similar highly oxidizing chemical solution.
To manufacture a flexible liquid crystal display, the thickness of TFTs and the device containing them needs to be cut down. The gate oxide film too needs to be thin for the same reasons, whilst maintaining insulation, through the formation of uniform film of high quality.
Also to manufacture a flexible liquid crystal display, the TFTs need to be formed on an organic substrate such as PET (polyethylene terephthalate). To this end, the TFTs must be fabricated at 200° C. or lower temperatures.
The high temperature oxidation methods and CVD described above however require conditions of at least 800° C. and 400° C. respectively to fabricate the gate oxide film. The methods are not suitable for TFT fabrication in the manufacture of the flexible liquid crystal display. In the high temperature thermal oxidation methods, the dopant diffuses due to heating at high temperatures, wiping out junctions close to the surface.
In CVD, an oxide film is deposited on the substrate. A uniform gate oxide film cannot be formed on a substrate with an irregular or curved surface. Furthermore, the obtained film is of inferior quality to the oxide film fabricated by direct oxidation, for example, in a thermal oxidizing solution. Therefore, CVD does not produce a highly reliable, completely insulating gate oxide film. The gate oxide film needs to be formed up to a sufficient thickness to prevent insulation breakdown as described above. This requirement arises from the non-uniformity of the gate oxide film.
CVD has other disadvantages too. The method uses dangerous SiH4, which ignites when released into the air. Many other gases used in the method need careful handling. CVD therefore requires large-scale facilities from gas introduction to venting. Hence, there is demand for a simple method to fabricate a high quality gate oxide film.
Therefore, if a high quality gate oxide film is formed uniformly, the gate oxide film can be further thinned down.
In conventional anodization, high voltage (typically, 100 V or higher) is needed to move silicon ions in the silicon substrate from the silicon substrate to the gate oxide film (silicon dioxide film). Specifically, in anodization, the silicon dioxide film (oxide film) on the surface of the silicon substrate in electrolyte grows when silicon ions (Si+) in the silicon substrate from the interface between the silicon substrate and the silicon dioxide film through the silicon dioxide film to the surface of the silicon dioxide film (interface between the silicon dioxide film and the electrolyte) and oxidized on the surface of the silicon dioxide film. As the silicon dioxide film forms and grows thicker, an increasingly large voltage needs to be applied across the silicon substrate. Nevertheless, excess voltage will cause insulation breakdown. It is therefore difficult to form a relatively thick, high quality silicon dioxide film.
In addition, in anodization, ions in the electrolysis solution contaminate the oxide film. It is rather difficult to obtain a high quality oxide film. For example, electrical properties are not sufficiently stable. Therefore, to ensure that the oxide film formed by anodization provides target quality, the oxide film needs to be thick, which means that anodization is also short of producing a high quality oxide film. If the oxide film has such a shape that the stress arising from volume expansion of the oxide film acts on the Si substrate, the stress retards the growth of the oxide film in some cases. Where the stress is concentrated, the oxide film in those parts is thinner than in other parts. The phenomenon leads to failure to form a uniform oxide film, degrades film quality, and allows leak current to occur.
TFTs in driver LSIs and related switching elements for the liquid crystal display are based on CG silicon (continuous grain silicon). CG silicon is fabricated by thermal annealing in which crystals collide to form angular projections. Therefore, the surface of CG silicon has a complex, irregular geometry. Technology is being demanded which is able to form uniform oxide film on such a complex surface.
Manufacture processes at low temperatures generally lead to poor reliability. For example, if TEOS oxidation (CVD), a popular approach in oxide film fabrication, is carried out at low temperature, film quality falls drastically, causing leak current. In short, low temperature leads to poor oxide film quality. Also, as mentioned above, it is difficult to form a uniform oxide film on a complex surface.
These examples demonstrate that the prevention of the deterioration of oxide film performance and reliability in low temperature manufacturing processes is the biggest issue in the development of flexible liquid crystal displays and like apparatus.
Meanwhile, improving insulating film performance is important in semiconductor devices, particularly in semiconductor integrated circuits based on MOS transistors, in which circuit elements are progressively scaled down for a higher degree of integration and higher density.
In these semiconductor integrated circuits, the gate insulating films in MOS transistors are usually fabricated with a “high temperature thermal oxidation method” which involves heating at or above 800° C. in dry oxygen, water vapor, or another oxidizing gas.
Well known methods for oxide film fabrication other than the high temperature thermal oxidation method include chemical vapor deposition (CVD), sputter vapor deposition, and plasma oxidation. In CVD, an organic silane, for example, tetra-ethoxy-silane (TEOS), is thermally decomposed at a few hundred degrees Celsius so that it deposits on a substrate to form an oxide film. In sputter vapor deposition, an oxide is formed by sputter vapor deposition. In plasma oxidation, the substrate surface is oxidized in plasma.
Tokukaihei 3-6826 discloses examples of anodization with which the substrate surface is oxidized to form an oxide film. Voltage is applied to a silicon substrate in an aqueous solution of hydrofluoric acid (electrolyte) to form a porous anode-reaction silicon film. The porous anode-reaction film is then subjected to anodization in an electrolyte in which silicon is anodized, for example, thick phosphoric acid.
In contrast, the inventor has suggested the formation of a thin oxide film on the surface of a silicon or other semiconductor substrate using an oxidizing chemical solution, such as thick nitric acid in Tokukai 2002-64093.
Ultrathin oxide films on the order of nanometers (nm) or even less is indeed fabricable after removing a natural oxide film from the silicon surface. It is however rather difficult to control the quality of the film so that it can be used as an insulating film in a semiconductor device, especially, to obtain one with low leak current density. To ensure voltage tolerance, the gate insulating film (oxide film) in the thin film transistor (TFT) and other applications needs to be relatively thick: a few nanometers (nm) or even thicker.
To form thin film transistors (TFTs) on a flexible substrate, for example, a polyethylene terephthalate (PET) substrate, for liquid crystal displays and other applications, the substrate needs to be kept not above 200° C. A low temperature manufacturing method is hence needed which still endows an insulating film with such high quality that it can be applied to TFT gate insulating films and like semiconductor devices.